Rectennas are antenna structures that intentionally incorporate rectifying elements in their designs.
Satellites are an integral part of modern communication systems, and their importance can be expected to grow in the coming years. As future generations of satellites with greater capabilities become possible, it is expected that they can take an even more active role in future military conflicts.
The design of present-day satellites often involves tradeoffs among such aspects as weight, power, and electronic capabilities. Each new electronic system adds weight, and must compete for power with other required systems such as station keeping. The limits of these tradeoffs are eased only gradually from one generation to the next, by the evolution of electronics, batteries, propulsion systems, and so on. Thus, developing new technologies that significantly expand the available design space is crucial to the enablement of satellites with radically improved capabilities over the present generation.
Power supply or generation is one area where revolutionary changes could significantly expand satellite capabilities. Presently, power sources are limited to solar panels or on-board power supplies. Solar panels require continuous exposure to the sun, or the use of batteries to supply power during periods of darkness. Any on-board power system such as a battery adds weight, which reduces the number of electronic systems that can be flown. Furthermore, a system of solar panels and/or on-board sources is best suited to continuous power at moderate levels, and cannot easily supply high-energy bursts without significant additional weight in order to collect and store, and then release the energy.
One way of providing a more flexible power source is to beam the power from a ground station 10 to a satellite 20, as illustrated in FIG. 1. This concept has been explored in the past, but in the opposite direction: beaming power to earth (which seemed attractive during the energy crisis). Sending power in the space to earth direction faces certain fundamental limits that make it impractical, but these limits are eased in the earth to space direction, leading to a system that is within the realm of possibility.
In addition to satellites, there are many other applications where beaming power could be important. For example, it is possible to replace hundreds of civilian cellphone base stations with a single zeppelin 20′, shown in FIG. 2, which could service a large metropolitan area 25 with mobile telephony, as well as such other services as “satellite” television. This would provide a low-cost alternative to satellites for many commercial wireless applications.
Furthermore, other applications include small UAVs (Unmanned Aerial Vehicles) that could be powered by beamed energy. See FIG. 3. As the size of a UAV is reduced, the amount of weight that it can carry limits its lifespan significantly. For example, 100-gram airplanes have been built, but their lifetime is limited to six minutes with currently available batteries. By beaming power to a micro-UAV 20″, it could stay aloft much longer. This would be useful for such applications as law enforcement, surveillance, hazardous site investigation, etc., in addition to the obvious military applications.
The embodiments of FIGS. 1–3 assume that the source of power is from a ground station 10. However, the source of power need not necessarily be terrestrial. The source of power could be airborne or even in space.
Any beamed power system must confront the fundamental limits summarized by the Friis transmission equation, which relates the total power transmitted to the gain, G, of the transmitting and receiving antennas, the distance between them, R, and the wavelength λ of the radiation used.
                              P          Rx                =                              G            Tx                    ⁢                                                    G                Rx                            ⁡                              (                                  λ                                      4                    ⁢                    π                    ⁢                                                                                  ⁢                    R                                                  )                                      2                    ⁢                      P            Tx                                              [        1        ]            
Assuming for simplicity that both antennas are circular, the gain of each is related to its diameter, D.
                    G        =                              (                                          π                ⁢                                                                  ⁢                D                            λ                        )                    2                                    [        2        ]            
If one assumes for the moment that very little power will be lost to spillover (this requirement can be relaxed) these equations can be combined to yield an expression for the required sizes of the transmitting and receiving antennas, as a function of their separation, and the wavelength of the radiation used. See FIG. 4, which depicts the geometry involved in equation 3, to determine the required diameters for the transmitting and receiving antennas.
                                          D            Tx                    ⁢                      D            Rx                          =                              4            π                    ⁢          R          ⁢                                          ⁢          λ                                    [        3        ]            
For a given separation, reducing the wavelength reduces the size requirements of the transmitter and/or receiver. One tempting solution is to use optical wavelengths, and beam power to space with a large earth-based laser. This has several drawbacks, including scattering by atmospheric turbulence and airborne particles, the typically low wall-plug efficiencies of lasers compared to microwave sources, and the losses in conversion back to DC by photovoltaic cells. Lasers may be viable alternatives for stationary, near-earth applications such as zeppelins, but not for moving applications, such as micro-UAVs. Their utility for satellites is questionable.
The next candidate wavelength range after optical (skipping terahertz frequencies, which are currently not feasible) is millimeter waves. In the 90–100 GHz range, the attenuation for a one-way trip through the atmosphere can be as little as 1 dB (See Koert, 1992, infra). Furthermore, efficient high-power sources are available, such as the gyrotron, which can produce as much as 200 kW of continuous power at millimeter wave frequencies, at an efficiency of 50% (See Gold, 1997, infra). For higher power applications, arrays of klystrons have been proposed that could produce tens of megawatts of power. These existing high-power sources suggest that it could be possible to temporarily supply a satellite with much higher power from the ground than can currently be produced in orbit. For comparison, the most powerful commercial satellite that is available, the Boeing 702, operates at 25 kW from on-board solar panels. These power sources would be more than adequate for airship applications, and the power required for micro-UAVs would only be on the order of watts.
The most significant engineering challenge for efficient earth to space power transmission is the design of the transmitting and receiving antennas. Fortunately, the receiver design is greatly simplified by the development of the rectenna, (See Brown, 1984, infra) which consists of an array having a rectifier diode at each element. Converting to DC directly at each antenna eliminates the requirement for a perfectly flat phase front, and permits the receiving aperture to take any shape. The transmitter must still produce a coherent beam, so a parabolic dish or other method of phase control is necessary. This is one reason why space to earth transmission is impractical. To illustrate the possibility of high-efficiency earth to space transmission, consider the following example.
Assume that 100 GHz radiation is to be used. The maximum transmitter gain is determined by the ability to accurately build a large dish with the necessary smoothness. The Arecibo dish, which operates at 10 GHz, is 300 meters in diameter. First, assume that a 100 GHz dish could be similarly built with a diameter of 30 meters.
Next, assume that a low-earth-orbit (LEO) satellite is utilized, at an altitude of 500 km. Using equation 3, the required receiver diameter for high transmission efficiency is about 60 meters. This can be compared to the Boeing 702 solar panel wingspan of 47 meters. Thus, structures of the required sizes can be built, both on earth and in space.
However, existing rectenna designs are not practical for space power applications because they require an enormous number of diodes to cover such a large area. For the example just described, one diode per half-wavelength at 100 GHz equates to 6 billion diodes. Using 12-inch wafers, and assuming an area of 1 mm square per diode, this represents the yield of 20,000 wafers; the weight and cost of the diodes alone would be prohibitive.
Another problem with space power applications using traditional rectenna designs is that the power density is too low to achieve significant efficiency. The efficiency, h, of a rectenna is related to the voltage across the diodes, VD, and the built-in diode voltage, Vbi (See McSpadden, 1998, infra).
                    η        ∝                  1                      1            +                                          V                bi                            /                              V                D                                                                        [        4        ]            
Designs with efficiencies as high as 90% have been demonstrated, [Strassner, 2002] but the power densities involved were much higher than one could expect to encounter in space. For the LEO example given above, the power density would be 6 mW/cm2, which corresponds to only 0.2 volts generated across each diode—on the order of the typical built-in voltage for a Schottky diode. The practical limitations of a space power system are thus the large number of diodes needed, and the low voltage generated across each diode. The efficiency could also be improved by placing each diode inside a high Q resonant structure, or by using diodes with lower built-in voltage. However, either of these solutions alone would not solve the problem of the large number of required diodes.
As such there is a need for lens-like structures that will allow the number of diodes to be reduced.
In terms of the prior art and a better understanding of the background to the present invention, the reader is directed to the following articles:    W. Brown, “The History of Power Transmission by Radio Waves”, IEEE Transactions on Microwave Theory and Techniques, vol. 32, no. 9, pp. 1230–1242, September 1984.    P. Fay, J. N. Schulman, S. Thomas III, D. H. Chow, Y. K. Boegeman, and K. S. Holabird, “High-Performance Antimonide-Based Heterostructure Backward Diodes for Millimeter-wave Detection”, IEEE Electron Device Lett. 23, 585–587 (2002).    S. Gold, G. Nusinovitch, “Review of High Power Microwave Source Research”, Review of Scientific Instruments, vol. 68, no. 11, pp. 3945–3974, November 1997.    P. Koert, J. Cha, “Millimeter Wave Technology for Space Power Beaming”, IEEE Transactions on Microwave Theory and Techniques, vol. 40, no. 6, pp. 1251–1258, June 1992.    H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming Light from a Subwavelength Aperture”, Science, vol. 297, pp. 820–822, Aug. 2, 2002.    J. McSpadden, L. Fan, K. Chang, “Design and Experiments of a High-Conversion-Efficiency 5.8 GHz Rectenna”, IEEE Transactions on Microwave Theory and Techniques, vol. 46, no. 12, pp. 2053–2060, September 1984.    J. N. Schulman and D. H. Chow, “Sb-Heterostructure Interband Backward Diodes,” IEEE Electron Device Lett., 21, 353–355 (2000).    D. Sievenpiper, J. Schaffner, H. Song, R. Loo, G. Tangonan, “Two-Dimensional Beam Steering Using an Electrically Tunable Impedance Surface”, IEEE Transactions on Antennas and Propagation, special issue on metamaterials, October 2003.    B. Strassner, K. Chang, “5.8 GHz Circularly Polarized Rectifying Antenna for Wireless Microwave Power Transmission”, IEEE Transactions on Microwave Theory and Techniques, vol. 50, no. 8, pp. 1870–1876, August 2002.    F. Yang, Y. Qian, T. Itoh, “A Uniplanar Compact Photonic Bandgap (UCPBG) Structure and its Applications for Microwave Circuits”, IEEE Transactions on Microwave Theory and Techniques, vol. 47, no. 8, pp. 1509–1514, August 1999.